It is well known to make flexible urethane foams by hot molding. The molding of any part starts with mixing and pouring from standard polyurethane foam-making equipment. Molds of the appropriate size, usually made of cast aluminum, must be heated to from about 100.degree. to 200.degree.F. before charging to prevent heat loss from the foaming mass. This technique, along with the proper use of a release agent, helps to provide reproducible parts having good properties. Another important factor is the quick heating or curing of the molded foam at a temperature of from about 300.degree. to 450.degree.F. for from about 3-20 minutes. Compression set, a significant property, is influenced by this heating step. Final cure is attained after aging at ambient (room) conditions. Although mechanical and heat transfer variables can be critical in the molding process, the formulation determines most end properties. The largest application for the hot-molded flexible urethane foam is in parts for automobiles, particularly seats.
For example, many of the automobile deep seat foams manufactured have been molded via the hot cure technique. Foams can be molded around steel wires which gives rigidity and a means to anchor the foam part to the car body. A piece of burlap which may become part of the foam during molding is used as an insulator between a flat S shaped spring and the foam cushion.
It has been observed that when a polyetherurethane low density flexible or semiflexible foam formulation containing polyvinyl chloride (PVC), ZnO and Sb.sub.2 O.sub.3 as described, for example, in U.S. Pat. No. 3,574,149 is used in a hot-molding foam process, many of the foams produced are defective in that they have areas or sections in which the foam crumbles or disintegrates when the mold is opened and the foam removed from the mold. In some instances the foam may appear satisfactory, but then it will readily crumble when a finger is pressed or run along the surface. Even though some foams are initially satisfactory, the system has been observed to change with time so that at the end of a given run many foams are unsatisfactory. Such foams cannot be repaired readily which results in considerable economic loss.
It has been observed that these defective areas occur largely on the surface of the foam and where, in most cases, the liquid foamable mixture discharged from the mixing head or nozzle has run down into the mold to form a puddle or puddles prior to foaming. While the blowing of the foams appears to be complete on opening the mold, the curing or crosslinking reaction appears not to have been completed. In other words, for some reason, the isocyanate-hydroxyl reaction went so far and stopped, the urethane foaming reaction was killed or the activity of the catalyst was inhibited or stopped.
It, therefore, is an object of the present invention to overcome the difficulties alluded to above and to provide a hot-molded flexible or semiflexible low density flame-retardant polyetherurethane foam free or essentially free of surface defects or crumbly areas or portions and/or in which the surface of the foam conforms faithfully with the inner surface of the mold and, also, to provide a method or process for making such foams.
These and other objects which are apparent to those skilled in the art from the following detailed description and examples are satisfied by this invention.